The technical field of this invention is the deployment of an occupant restraint in a motor vehicle during a vehicle crash event.
Occupant restraint deployment controls typically provide a xe2x80x9csmartxe2x80x9d crash sensor, consisting of an accelerometer with very sophisticated signal processing and logic, in the passenger area of a vehicle, supplemented by one or more simple satellite crash sensors in other locations providing additional information to the process. But these controls continue to develop in capability and complexity. Two areas of development that are rapidly progressing are the use of multiple stages of restraint deployment for different levels of crash severity and increased sophistication in satellite crash sensors located outside the vehicle passenger area. These trends have led to xe2x80x9csmartxe2x80x9d satellite sensors having multi-stage deployment signal capability.
One of the satellite sensors for such systems is located in a frontal xe2x80x9ccrush zonexe2x80x9d of the vehicle, where it provides an earlier look at the accelerations produced by a crash event, but is also more sensitive to misuse, rough road or deer hit events that produce initial accelerations similar to those of a crash but for which occupant restraint deployment is not desired. A sophisticated satellite sensor designed for this location might derive a velocity measure by integrating an accelerometer signal and compare the velocity measure to a dynamic or time dependent threshold, which varies, usually increasing, as a function of time relative to the beginning of a sensed possible crash event. Several such dynamic thresholds are shown in the curves of FIG. 3, in which three possible crash event velocity curves are superimposed on two velocity thresholds as a function of time. The two thresholds are shown as solid lines: curve 4 is the lower threshold testing for a lower stage deployment signal; and curve 5 is the higher threshold testing for a higher stage deployment signal. A desired trigger time DTT is shown on the time axis. A velocity curve must cross a threshold within time DTT to produce deployment, and the level of desired deployment signaled is that of the highest threshold crossed within the time. The velocity curves are broken lines. Curve 7 exceeds the lower curve 4 before time DTT but never exceeds curve 5. It thus produces a lower stage deploy signal. Curve 8 also exceeds curve 7 but not curve 8 within time DTT and thus also generates a lower stage deploy signal. But the overall behavior of curve 8 is that of a higher severity crash which would, if identified earlier, call for a high level deployment. Curve 8 xe2x80x9cfoolsxe2x80x9d the system because, although it produces an initial significant velocity, it temporarily loses velocity in the frontal xe2x80x9ccrush zonexe2x80x9d location of the satellite sensor as the metal starts to crumple. The high velocity is postponed until after this metal crushing has occurred.
An observer will recognize that the problem described above could be avoided if curves 7 and 8 were lowered at the earliest times (the extreme left portions) so as to catch the initial xe2x80x9chumpxe2x80x9d of curve 8. But this would then also provide a second stage deploy signal for the event of curve 9, representing a deer hit or similar event that produces an initial high acceleration but does not accumulate much velocity and requires no deployment at all. Thus, even dynamic thresholds have their limitations in the complex task of multi-stage restraint deployment.
In addition, dynamic thresholds are not particularly efficient in their use of limited computer memory resources. A constant threshold requires memory space for only a single number, but a dynamic threshold requires memory space for a series of numbers or for the code required for storing an equation and calculating the required numbers.
The apparatus and method of this invention provide a satellite crash sensor for a motor vehicle occupant restraint system that provides multi-stage deploy signaling in a manner that detects a second stage deploy crash event having an initial velocity rise followed by a loss of velocity that delays the continuation of the velocity rise, and does so in a manner that is efficient with respect to computer memory resources.
The apparatus and method of this invention use an accelerometer to provide an acceleration signal and derive a velocity value therefrom. The initiation of a possible crash event is detected and a clock count indicating a time progression into the event is initiated. Means are provided for storing a constant second stage threshold value and data defining a first stage threshold that varies as function of the clock count. Additional means are effective only for a time indicated by a predetermined value of the clock count for comparing the velocity value with the second stage threshold and storing a second stage datum if the velocity exceeds the second stage threshold value. Finally, means are provided for comparing the velocity value with a clock count determined value of the first stage threshold and alternatively (1) generating a second stage deploy signal if the velocity value exceeds the value of the first stage threshold and the second stage datum is stored, (2) generating a first stage deploy signal if the velocity value exceeds the value of the first stage threshold and the second stage datum is not stored, and (3) generating no deploy signal if the velocity value does not exceed the first stage threshold. Preferably, for at least a portion of the predetermined clock count, the clock determined value of the first threshold exceeds the second threshold value.